Neza-Chalco-Itza, Mexico City

One of Mexico City's many barrios, Neza-Chalco-Itza is the largest slum in the world with roughly four million people. Slums in Mexico City began growing in the early 1900s when the railroad brought new industry -- as well as new industrial zones -- to the city.

As with many slums, most people in Neza-Chalco-Itza live illegally on unauthorized land. However, unlike in most areas, some of Mexico City's slum dwellers also live in vecindades, former mansions abandoned by wealthy families that have been converted into low-income rental apartments. About 10 percent of all the residents of the Mexican capital live in these buildings.